Process for the preparation of 2-hydroxy-naphthalene-6-carboxylic acid

ABSTRACT

2-Hydroxy-naphthalene-6-carboxylic acid or its dipotassium salt is obtained in a good yield and purity without substantial formation of the isomeric compound 2-naphthol-3-carboxylic acid by reacting potassium β-naphtholate with potassium carbonate in the presence of carbon monoxide at a temperature above 260° C. in potassium formate as the solvent.

The present invention relates to the technical field of intermediateproducts which can be employed, for example, for the synthesis of azodyestuffs and polyester base materials.

2-Hydroxy-naphthalene-6-carboxylic acid is not only a useful synthesisunit for pharmaceuticals, textile auxiliaries and dyestuffs (cf., forexample, European Patent Application Publication No. 0,292,955A), but inparticular is also an important monomer for the preparation of liquidcrystalline high-performance plastics and fibers having outstandingproperties (cf. U.S. Pat. No. 4,393,191).

2-Hydroxy-naphthalene-6-carboxylic acid is synthesized industrially by aprocedure analogous to the so-called Kolbe-Schmitt reaction, i.e. byreaction of the potassium salt of β-naphthol with carbon dioxide, underpressure at 200°-300° C. (cf., for example, U.S. Pat. Nos. 1,593,816,4,329,494 and 4,287,357). However, the procedures described in thesepublications have some industrial disadvantages; considerable amounts ofdecomposition products, such as tars and resins, are formed which, likethe 2-hydroxy-naphthalene-3-carboxylic acid and2-hydroxy-naphthalene-3,6-dicarboxylic acid formed in side reactions,can be removed only with difficulty. Attempts have indeed been made toimprove the reaction procedure and to reduce the formation of theintermediate products by the procedures described in European PatentApplication Publications Nos. 0,053,824 and 0,081,753 by carrying outthe process in a solvent, preferably kerosene. In spite of thisadvantage over the solvent-free Kolbe-Schmitt reaction, however, thesetwo process variants still have the fundamental disadvantage that ineach case 1 mol of β-naphthol remains unreacted in the reaction mass permole of 2-hydroxy-naphthalene-6-carboxylic acid formed, which is whyeven in the best possible reaction procedure the yield is limited to amaximum of 50% in accordance with the equation: ##STR1## In thepreparation of the isomeric compound 2-hydroxy-naphthalene-3-carboxylicacid, it has already been proposed to avoid this disadvantage ofincomplete conversion of the β-naphthol into naphtholcarboxylic acid bycarrying out the reaction of potassium β-naphtholate with potassiumcarbonate in the presence of carbon monoxide in accordance with theequation: ##STR2## (see British Patent Specification 1,155,776).However, if attempts are made to modify this procedure with the aim ofpreparing 2-hydroxy-naphthalene-6-carboxylic acid by varying thereaction conditions of temperature and carbon monoxide pressure, thesynthesis of this 6-carboxy derivative remains unsuccessful.

It has now been found that 2-hydroxy-naphthalene6-carboxylic acid isobtained in a good yield and purity in a surprising manner if thereaction of the potassium β-naphtholate with potassium carbonate andcarbon monoxide is carried out at a temperature above 260° C., such asat a temperature between 270° C. and 360° C., preferably between 280°and 320° C., and under a carbon monoxide pressure of above 10 bar, suchas under a CO pressure of 50 to 150 bar, preferably between 70 and 140bar, and in particular under a CO pressure between 80 and 120 bar, inpotassium formate as the solvent (diluent).

The reaction according to the invention in potassium formate as thesolvent or diluent (potassium formate melts above 167.5° C. to give aclear liquid) is carried out in accordance with the equation: ##STR3##The formate used as the solvent (diluent) is employed in at least thesame molar amount as the potassium β-naphtholate starting substance; theamount of potassium formate used is not critical and can vary withinwide limits. However, it is advantageous to employ a relatively largeamount of potassium formate as the solvent (diluent), such as, forexample, 2.5 to 18 times, preferably 6 to 15 times, the amount by weightof the potassium β-naphtholate employed, in order to ensure the bestpossible thorough mixing of the reactants potassium β-naphtholate andpotassium carbonate, dissolved in the potassium formate, and the gaseouscarbon monoxide at the phase boundaries. As with any reaction between agas and a liquid phase to be carried out in industry, the customary goodmeasures for thorough mixing are of course to be taken here, such as,for example, by using high-intensity stirrers, such as turbine stirrers.

The potassium formate employed as the solvent (diluent) not only has theeffect of a high selectivity of the reaction to form the potassium saltof 2-hydroxy-naphthalene-6-carboxylic acid, but also has the advantagethat it is not a foreign product in the reaction batch, but is identicalto the by-product of the reaction. All of the potassium formate cantherefore be employed again in later reactions, or carbon monoxide isobtained from the excess of this product by heating with concentratedsulfuric acid and can be recycled into a later reaction.

The potassium carbonate also employed in the procedure according to theinvention is employed in the reaction in at least the same molar amountas the potassium β-naphtholate. The molar ratio between potassiumβ-naphtholate and potassium carbonate is as a rule between 1:1 and1:1.5.

When the synthesis has ended, the batch can be worked up in various waysand the 2-naphthol-6-carboxylic acid formed can be isolated. Onepossibility is to dissolve the reaction product in water, to bring thepH to 7 with concentrated sulfuric acid and to filter off the unreactedβ-naphthol which has precipitated. The filtrate is brought to a pH of 1by means of sulfuric acid and the crude2-hydroxy-naphthalene-6-carboxylic acid which has now precipitated isseparated off. Fine purification is carried out by procedures analogousto known procedures, for example by redissolving under pressure in wateror by purification with 1,4-dioxane (in this context cf., for example,German Offenlegungsschrift 3,800,989).

The following examples serve to illustrate the invention. The parts areparts by weight and the percentage data are percentages by weight,unless indicated otherwise. Parts by weight bear the same relation toparts by volume as the kilogram to the liter.

EXAMPLE 1

10 parts of potassium β-naphtholate, together with 7.6 parts ofpotassium carbonate and 75 parts of potassium formate, are introducedinto a stainless steel autoclave provided with a stirrer, and the entiremixture is freed from residual moisture at 230° C. in the course of 5hours, while stirring. Carbon monoxide is then passed in at roomtemperature under a pressure of 50 bar and the reaction is carried outunder a carbon monoxide pressure of about 95 bar at 280° C. in thecourse of about 5 hours, while stirring intensively.

The reaction batch is then cooled, the autoclave is depressurized, thereaction product is dissolved in water, the pH is brought to 7 withconcentrated sulfuric acid, the unreacted β-naphthol which hasprecipitated is filtered off, the filtrate is brought to a pH of 1 bymeans of sulfuric acid and the 2-hydroxy-naphthalene-6-carboxylic acidwhich has now precipitated is isolated. The 2-naphthol-6-carboxylic acidcan be separated off from the by-products out of the crude product inthe customary manner and can thus be obtained in a pure form.

Yield: 34% of theory, based on the β-naphthol as the starting substance.

The 2-naphthol-3-carboxylic acid is formed as a by-product to the extentof 5%.

EXAMPLE 2

The procedure is according to the procedure of Example 1, with thedifference that the reaction is carried out at 300° C. using 125 partsof potassium formate. The 2-hydroxy-naphthalene-6-carboxylic acid isobtained in a yield of 40% of theory, in addition to 5.3% of2-naphthol-3-carboxylic acid.

EXAMPLE 3

The procedure is according to the procedure described in the aboveexamples under a carbon monoxide pressure of 100 bar and at atemperature of 320° C. over a period of about 5 hours, using 10 parts ofpotassium β-naphtholate, 7.6 parts of potassium carbonate and 75 partsof potassium formate. After working up the reaction mixture,2-hydroxy-naphthalene-6-carboxylic acid is obtained in a yield of 37.5%of theory, in addition to 5.7% of 2-naphthol-3-carboxylic acid.

EXAMPLE 4

250 parts of potassium β-naphtholate, 190 parts of potassium carbonateand 3,500 parts of potassium formate are introduced into a stainlesssteel autoclave (about 5,000 parts by volume) provided with a diskstirrer, and the entire mixture is heated at 230° C. in the course of 5hours, while stirring, for complete removal of the water (moisture)contained in the starting substances. Carbon monoxide is then introducedbelow the stirrer up to a pressure of 85 bar, the temperature of thebatch is increased to 300° C. and the reaction is continued under acarbon monoxide pressure of about 100 bar for about a further 5 hours ata stirrer speed of about 1,100 revolutions/minute.

The reaction product is then worked up and purified by the proceduredescribed in Example 1. The 2-hydroxy-naphthalene-6-carboxylic acid isobtained in a yield of 64% of theory.

We claim:
 1. A process for the preparation of2-hydroxy-naphthalene-6-carboxylic acid or its dipotassium salt, whichcomprises reacting potassium β-naphtholate with potassium carbonate andcarbon monoxide at a temperature above 260° C. and under a carbonmonoxide pressure above 10 bar in 2,5- to 18-times the amount by weight,calculated with respect to the amount of the potassium β-naphtholate, ofpotassium formate as the solvent or siluent, and optionally convertingthe dipotassium salt thus formed into 2-hydroxy-naphthalene-6-carboxylicacid.
 2. The process as claimed in claim 1, wherein the reaction iscarried out at a temperature between 280° and 320° C.
 3. The process asclaimed in claim 1, wherein the reaction is carried out under a carbonmonoxide pressure between 70 to 140 bar.
 4. The process as claimed inclaim 2, wherein the reaction is carried out under a carbon monoxidepressure between 70 and 140 bar.
 5. The process as claimed in claim 2,wherein the reaction is carried out under a carbon monoxide pressurebetween 80 and 120 bar.
 6. The process as claimed in claim 1, whereinsaid dipotassium salt is the product of said process.
 7. The process asclaimed in claim 1, wherein said dipotassium salt is converted to2-hydroxynaphthalene-6-carboxylic acid, so that2-hydroxynaphthalene-6-carboxylic acid can be isolated as the product ofsaid process.